<br>Anyway, I believe that the behaviour of a certain towgs84 string is easy to test with cs2cs itself: just do the transormation to one direction (e.g. from WGS84) and convert the resulting coordinates back to the original datum using exactly the same parameter string. If the resulting coordinates equal to the original ones, no negation is needed. Right?<br>
<br>ilumas <br><br><br><br><div class="gmail_quote">On Fri, Apr 23, 2010 at 12:41 PM, Ilumas <span dir="ltr"><<a href="mailto:muginaa@gmail.com" target="_blank">muginaa@gmail.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">Thanks Mikael,<br><br>what if the Method is:<br><br><span> </span> <div>Method [Geocentric translations (geog2D domain)]<br>
<br>and the Sign reversible field value is YES for all three parameters?<br>
<br><br>best regards,<br><font color="#888888"><br>Ilumas<br><br><br><br><br></font></div><div><div></div><div><br><br><div class="gmail_quote">On Fri, Apr 23, 2010 at 11:37 AM, Mikael Rittri <span dir="ltr"><<a href="mailto:Mikael.Rittri@carmenta.com" target="_blank">Mikael.Rittri@carmenta.com</a>></span> wrote:<br>
<blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;"><br>
About the rotation sign conventions, I would<br>
add that calling the two conventions "European"<br>
and "American" is not helpful, since there is not<br>
much agreement about which convention is which!<br>
(Both conventions are used in Europe, and there<br>
is no obvious geographic pattern.)<br>
<br>
The terms "left-handed" and "right-handed" are<br>
just as bad.<br>
<br>
It is better to say that cs2cs uses the sign<br>
convention known as the<br>
<br>
Position Vector Transformation.<br>
<br>
If you look up the +towgs84 parameters on <a href="http://www.epsg-registry.org" target="_blank">www.epsg-registry.org</a><br>
(filter by Type: Coordinate Transformation), you will see the text<br>
<br>
"Warning: CARE! Ensure application uses this method! See Guidance Note<br>
7-1"<br>
<br>
above the parameter values. This means that you<br>
should scroll down _below_ the parameter values,<br>
where you find the Method. If it says:<br>
<br>
+ Method [Position Vector transformation]<br>
<br>
you can use the parameters directly in cs2cs.<br>
But if it says<br>
<br>
+ Method [Coordinate Frame Rotation]<br>
<br>
you should reverse the signs of the three<br>
rotation parameters.<br>
<br>
Best regards,<br>
<font color="#888888">--<br>
Mikael Rittri<br>
Carmenta AB<br>
SWEDEN<br>
<a href="http://www.carmenta.com" target="_blank">www.carmenta.com</a><br>
</font><div><div></div><div><br>
-----Original Message-----<br>
From: <a href="mailto:proj-bounces@lists.maptools.org" target="_blank">proj-bounces@lists.maptools.org</a><br>
[mailto:<a href="mailto:proj-bounces@lists.maptools.org" target="_blank">proj-bounces@lists.maptools.org</a>] On Behalf Of Frank Warmerdam<br>
Sent: den 21 april 2010 18:14<br>
To: PROJ.4 and general Projections Discussions<br>
Subject: Re: [Proj] cs2cs's +towgs84 usage<br>
<br>
Ilumas wrote:<br>
> Dear list,<br>
><br>
> could you kindly confirm that the cs2cs's +towgs84 parameters remain<br>
> the same for a certain datum regardless of the direction of the<br>
> transformation. Some people (e.g.<br>
> <a href="http://n2.nabble.com/Datum-Shift-WGS84-to-Arc1960-not-working-propberl" target="_blank">http://n2.nabble.com/Datum-Shift-WGS84-to-Arc1960-not-working-propberl</a><br>
> y-td3410479.html)<br>
> <<a href="http://n2.nabble.com/Datum-Shift-WGS84-to-Arc1960-not-working-propber" target="_blank">http://n2.nabble.com/Datum-Shift-WGS84-to-Arc1960-not-working-propber</a><br>
> ly-td3410479.html>seem to suggest that the values need to be negated<br>
> when going FROM wgs84 TO something else, but this seems a bit strange<br>
> to me. Unfortunately all the examples in the man pages seem to be<br>
> about changing the coordinates TO wgs84...<br>
<br>
Ilumas,<br>
<br>
The parameters are always expressed in terms of what it takes to<br>
transform to WGS84. If PROJ is transforming from WGS84 it will<br>
internally reverse the sense of the transformation.<br>
<br>
Keep in mind there is also an issue with the handedness of the<br>
parameters.<br>
Some folks, mostly european, express the parameters with a different<br>
handedness. The scripts that generate the "epsg" init file already know<br>
about this case and convert to the usual PROJ.4/North American<br>
handedness but if you are researching parameters yourself from various<br>
sources you may need to be aware of it.<br>
<br>
Best regards,<br>
--<br>
---------------------------------------+--------------------------------<br>
---------------------------------------+------<br>
I set the clouds in motion - turn up | Frank Warmerdam,<br>
<a href="mailto:warmerdam@pobox.com" target="_blank">warmerdam@pobox.com</a><br>
light and sound - activate the windows | <a href="http://pobox.com/%7Ewarmerdam" target="_blank">http://pobox.com/~warmerdam</a><br>
and watch the world go round - Rush | Geospatial Programmer for Rent<br>
<br>
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